1. Field of the Invention
This invention relates to a corona discharge device for electrographic apparatus which makes use of a drum-shaped photosensitive screen.
2. Description of the Prior Art
A corona discharge device heretofore known in general, comprises a corona discharge wire, a shield electrode surrounding the corona discharge wire and having an opening and a field electrode opposed through the opening to the corona discharge wire. Between the field electrode and the corona discharge wire is applied a high voltage and both the shield electrode and field electrode are connected to ground. A flow of corona ions is directed to the shield electrode and to the field electrode to charge a record sheet or the like placed on the field electrode.
An electric current produced by the flow of corona ions directed to the field electrode is effective to charge the record sheet, while an electric current produced by the flow of corona ions directed to the shield electrode is an ineffective current. Experimental tests have shown that a current produced by a high voltage source mainly consists of the ineffective current and hence the effective current is very small. As a result, the conventional corona discharge device is extremely low in current utility efficiency.
In addition, since the shield electrode is connected to ground, a high voltage applied to the corona discharge wire results in spark discharge between the corona discharge wire and the shield electrode. As a result, the highest voltage applicable to the corona discharge wire is limited by the distance between the corona discharge wire and the shield electrode. If it is necessary to obtain a large effective current, the distance between the corona discharge wire and the shield electrode must be made great, thereby making the corona discharge device large in size.
In an electrographic apparatus which makes use of a drum-shaped photosensitive screen, in the case of using the corona discharge device for the purpose of transferring an electrostatic latent image produced on the photosensitive screen onto a dielectric coated record sheet, there is the risk that an image consisting of dots and produced on the dielectric coated record sheet will be subjected to strain or the dots will be enlarged by the diverging flow of corona ions directed to the field electrode and hence increasing the width of the imagewise charged area on the field electrode. As a result, it is necessary to reduce the electric source voltage applied to the corona discharge device and hence to reduce the recording speed in the case of transferring the electrostatic latent image on the photosensitive screen onto the dielectric coated record sheet. As a result, provision must be made for a corona discharge device which can produce a flow of corona ions having a high density.